This invention relates generally to turbine engines, and more specifically to protective environmental coatings placed on turbine engine components such as turbine blades and vanes.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. A high pressure turbine (HPT) follows the combustor and extracts energy from the combustion gases for powering the compressor. A low pressure turbine (LPT) follows the HPT and extracts additional energy from the combustion gases for powering an upstream fan in an aircraft turbofan engine application, or powers an external drive shaft for marine and industrial applications.
The turbines are arranged in stages including a stationary turbine nozzle having a row of vanes which direct the combustion gases into a corresponding row of turbine rotor blades. Each vane has an airfoil configuration extending radially in span between inner and outer bands which bound the combustion gases.
Each turbine blade includes an airfoil extending radially outward in span from an airfoil root at an integral platform. An integral blade shank extends between the platform and an integral dovetail for mounting the blade in a corresponding dovetail slot in the perimeter of a supporting rotor disk. The platform defines the inner boundary for combustion gases, and the radially outer tip of the airfoil is spaced closely adjacent to a surrounding turbine shroud that defines the outer boundary for the combustion gases. The shank supports the mechanical loads from the airfoil and platform and transfers these mechanical loads to the blade dovetail. The shank has interior passages in it which are in flow communication with the cooling passages inside the airfoil. The shank interior passages receive cooling air through passages in the blade dovetail and channel the cooling flow into the airfoil cooling circuits.
The corresponding airfoils of the vanes and blades in each turbine stage have generally concave pressure sides and generally convex suction sides extending axially in chord between opposite leading and trailing edges for efficiently turning the combustion gases and extracting energy therefrom during operation. The differently shaped opposite sides of the airfoils therefore effect different velocity and pressure distributions thereover, and correspondingly experience different heat loads from the combustion gases in highly complex three dimensional (3D) distributions.
The first stage turbine nozzle and blades first receive the hot combustion gases from the combustor and therefore have the greatest heat loads of the various turbine stages. Accordingly, the vanes and blades are typically cast from state of the art superalloy metals which have enhanced strength at elevated temperature for maximizing the useful life thereof during operation. In conventional engines, the turbine vanes and blades are made of nickel based superalloys, and can operate at temperatures of up to about 1900-2100 Deg. F. A protective layer or a metal/ceramic thermal barrier coating (TBC) system is sometimes applied to the airfoil, which acts to protect the base substrate metal of the component.
The blade airfoil and shank are hollow and include corresponding internal cooling circuits therein which receive a portion of the pressurized air bled from the compressor for cooling thereof during operation. The internal cooling channels located inside the blade shank typically include multiple radial channels defined by corresponding radial partition walls. The internal cooling circuits in the airfoil have multiple radial channels having walls that bridge the pressure and suction sides of the airfoil. The pressure and suction sides of the airfoil typically include radial rows or columns of film cooling holes extending transversely through airfoil walls.
The gas turbine blade or vane may be operated in a highly aggressive environment that may cause deterioration of the component in service. The environmental damage may be in various forms, such as particle erosion, different types of corrosion, and oxidation, and complex combinations of these damage modes, in the hot combustion gas environment. The rate of environmental damage may be lessened somewhat with the use of coatings comprising suitable protective layers.
In conventional turbine engine components aluminide coatings have been used in the internal passages of turbine blades and vanes to avoid failures from internal oxidation of the bare nickel superalloy base material. Although turbine blade alloys having greater oxidation resistance have been developed, these newer alloys may not possess adequate hot corrosion resistance. It is known in the art that oxidation of the parent material in the cooler internal blade passages is usually not very significant. However, under certain conditions hot corrosion in the cooler internal shank cavities may occur if the protective environmental coating does not provide sufficient protection against corrosive environments.
Conventional turbine engine components are typically made from nickel based superalloys. Aluminide environmental coatings are sometimes used in these conventional turbine engine components to protect the internal passages from oxidation and hot corrosion. Aluminide coatings are relatively more brittle as compared to the nickel based superalloy base material on which they are applied. Due to the brittle nature of aluminide coatings, cracks may initiate in the internal passages of turbine blades, especially in cooler and thicker areas of aluminide coating such as the blade shank. Therefore, in the relatively cooler locations of the interior passages of the turbine blade it is desirable to have an environmental coating that does not develop cracks.
Accordingly, it would be desirable to have a turbine blade having a ductile environmental coating to protect the relatively cooler internal passages from hot corrosion.